"Is that a good plant?" a woman asked me, pointing to a trailside flower during a nature walk.

I paused and smiled.

"I mean," she stuttered, sensing that the answer was not going to be a simple yes or no, "is it a wildflower or … a weed?" Ah, yes, the weed-or-wildflower debate was about to unfold.

'Good' and 'bad'

"Weed" is a word that everyone knows, but few people give much thought to what it really means. The concept of a weed is rooted in the belief that some plants are good and some plants are bad. Can plants be good? Can they be bad? Can they be both?

The good plant/bad plant dialectic is as old as humankind. Adam and Eve were out weeding the garden, no doubt, when the Roundup rep came to sell them herbicide.

Originally, bad plants were the ones that killed you or made you sick. Good plants were the ones that you could eat or turn into cool things like boats and zithers and spears.

Over time, as rudimentary agriculture developed, the bad plant category came to include those that prevented the good ones from thriving. The good guy category took on medicinal plants.

Other plants were decreed good simply because they made the heart happy. Maybe they were pretty or smelled good.

Somewhere in the mix of the good and the bad plants were the ones that took you closer to the gods in strange, new universes.

Over time, these psychotropic plants slid along the scale from good to bad and everywhere in between.

There have always been plants that people have liked, and plants that people have disliked. But liking something is one thing; pronouncing it good or bad is another.

Can a marigold be good? Does an oak tree have the capacity to be bad? What makes crab grass so diabolical and baby's-breath so angelic? Being the judge of such things implies some serious chutzpah. Think about it: if the tables were turned, what would creeping Charlie think about us?

Different definitions

Philosophers steeped in theories of environmental ethics bring up things like intrinsic value, moral standing, and consciousness. Most of us laypeople don't engage in such academic discussions, but if pressed, we would agree that a plant has neither good nor ill intent.

It's just doing its thing. Its thing is surviving and reproducing.

In other words, that goldenrod plant is really not out to get you. The dandelion is not plotting to make your life miserable. These plants share the same mission in life that all organisms have: grow, prosper, and produce healthy kids.

Plants that are really, really good at doing their thing -- like ragweed or crab grass or kudzu -- can get in the way of us doing our thing. Breathing, in the case of ragweed. Gardening, in the case of crab grass. Growing trees, in the case of kudzu. This is where the concept of weed comes in.

I asked the person on the hike what she thought a weed is. The others in the group chimed in the discussion. The consensus was, "A weed is a plant we don't like." Fair enough. "It's a plant that shouldn't be there." I pressed the pause button.

Difficulties arise as soon as the verb "should" slips in. Should and shouldn't are strong words. Weed, therefore, is a judgment call.

Consider the lilies-of-the-valley. They neither toil nor spin, but they pop up in my woods uninvited, and I remove them as soon as I see them. They're pretty, but invasive. Left alone, they would take over the native plants and I'd have nothing but lilies-of-the-valley. I don't think they're bad, nor do I hate them. I would prefer biological diversity.

What one person considers a weed in his garden may not be a weed in his neighbor's garden. Similarly, what a farmer considers a weed in her field may be different from what an ecologist considers a weed in the prairie.

What can be used as lifesaving medicine can also be used, with a tiny adjustment in dosage, to kill somebody. Which plant should be allowed to live where, and why?

What scientists say

Time to turn to the scientists. The objective, ecological definition of weed is: a pioneer plant that thrives in disturbed areas.

"Pioneer" refers to the first plants that move into an area. They're usually annuals or biennials that produce prodigious amounts of seeds. Their life cycle is fast and furious -- the better to get established in uncertain territory.

"Disturbed areas" are those that have experienced radical change, either natural or human-made. Natural disturbance can be a tornado tearing through a maple forest, or a hurricane ravaging a mangrove swamp.

Plant popularity

Plant popularity ebbs and flows, so "weed" is a moving target.

Take milkweeds, for example. These native plants are enjoying a surge in popularity, thanks to educational campaigns connecting milkweeds with monarch butterflies. But not too long ago, milkweeds were dismissed as agricultural weeds and zapped with herbicide along with every other roadside renegade.

Yet, in the 1940s milkweeds were a valuable resource in the war effort. Children were paid a nickel for every bag of milkweed pods they collected. The milkweed "fluff" was used in life jackets and parachutes for the troops overseas.

So, is milkweed a good plant or a bad plant? Is it a weed or a wildflower? It depends.

From a plant's point of view, this must seem very arbitrary, indeed.

The milkweed is doing nothing differently than it ever did.

The multiflora rose by any other name would smell as sweet.

Purple loosestrife was introduced as a garden ornamental but is now a noxious weed in wetlands.

In other words, a weed is a weed only in our heads.

The question, "Is that a good plant?" started out as a simple enough inquiry on a nature hike. There are lots of ways to answer the question, and the answers are always subjective.

Ralph Waldo Emerson wrote, "A weed is a plant whose virtues have not yet been discovered." May we see the virtues of our botanical kin as we tend to our gardens this summer.

• Valerie Blaine is a naturalist for the Forest Preserve District of Kane County. You may reach her at blainevalerie@kaneforest.com.

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